类型定义

types/react 类型定义中对useRef函数定义了三次（两次重载）

 /**
     * `useRef` returns a mutable ref object whose `.current` property is initialized to the passed argument
     * (`initialValue`). The returned object will persist for the full lifetime of the component.
     *
     * Note that `useRef()` is useful for more than the `ref` attribute. It’s handy for keeping any mutable
     * value around similar to how you’d use instance fields in classes.
     *
     * @version 16.8.0
     * @see https://reactjs.org/docs/hooks-reference.html#useref
     */
    function useRef<T>(initialValue: T): MutableRefObject<T>;
    // convenience overload for refs given as a ref prop as they typically start with a null value
    /**
     * `useRef` returns a mutable ref object whose `.current` property is initialized to the passed argument
     * (`initialValue`). The returned object will persist for the full lifetime of the component.
     *
     * Note that `useRef()` is useful for more than the `ref` attribute. It’s handy for keeping any mutable
     * value around similar to how you’d use instance fields in classes.
     *
     * Usage note: if you need the result of useRef to be directly mutable, include `| null` in the type
     * of the generic argument.
     *
     * @version 16.8.0
     * @see https://reactjs.org/docs/hooks-reference.html#useref
     */
    function useRef<T>(initialValue: T|null): RefObject<T>;
    // convenience overload for potentially undefined initialValue / call with 0 arguments
    // has a default to stop it from defaulting to {} instead
    /**
     * `useRef` returns a mutable ref object whose `.current` property is initialized to the passed argument
     * (`initialValue`). The returned object will persist for the full lifetime of the component.
     *
     * Note that `useRef()` is useful for more than the `ref` attribute. It’s handy for keeping any mutable
     * value around similar to how you’d use instance fields in classes.
     *
     * @version 16.8.0
     * @see https://reactjs.org/docs/hooks-reference.html#useref
     */
    function useRef<T = undefined>(): MutableRefObject<T | undefined>;

第二个重载返回的RefObject 对象中current属性是只读属性

第一次定义

function useRef<T>(initialValue: T): MutableRefObject<T>;
如果函数调用匹配到这个定义，则 ref.current 类型只能是initialValue类型
初始值为boolean，没有传入T，则类型推断T为boolean，则始终只能是boolean类型

第二次定义（重载）

function useRef(initialValue: T|null): RefObject;

current值可以是T或者null 两种类型, 常见于获取dom元素
因为可能为null，所以确定存在，可以使用！非空断言

还有一种非空断言写法如此，Ref对象也改变了,从RefObject 变成MutableRefObject了
并且匹配到第一次定义（目前这个写法还没完全搞明白）

第三次定义（重载）

function useRef<T = undefined>(): MutableRefObject<T | undefined>;

自定义hook—useRefObject

import { useRef } from 'react'
export function useRefObject<T>(t: T) {
  const ref = useRef<T>(t)
  ref.current = t
  return ref
}

使用这个hook，除了初始化useRef，每次render都会执行一遍ref.current = t, 那它的应用场景就应该是current值依赖别的状态。
比如说，依赖到props值，当props值变化时，也要同步改变。正常要使用useEffect

const ref = useRef(props.parmas1)
useEffect(() => {
  ref.current = props.parmas1
}, [props.params1])

但是使用useRefObject 就可以省略useEffect.